WMS label not triggered when using "Union" in the custom sql field

Hi we are facing an issue. User want to print WMS label even if the custom sql return no rows. So we are using "UNION" Statement. But because of this label are not triggering. Anyone please help. custom label sql is as below

SELECT mci.customer_item_number

FROM wms_label_Requests wlr,

  MTL_CUSTOMER_ITEM_XREFs xref,

  MTL_CUSTOMER_ITEMS mci,

  mtl_system_items_b msi

WHERE wlr.inventory_item_id     = msi.inventory_item_id

AND wlr.customer_id             = mci.customer_id

AND xref.customer_item_id       = mci.customer_item_id

AND xref.INVENTORY_ITEM_ID      = MSI.INVENTORY_ITEM_ID